We've all heard that bee populations are dwindling. Far less clear, however, is what's actually causing the slump. Now, two studies have provided some of the most compelling evidence to date that a popular class of insecticides may be contributing strongly to the collapse.

Scientists call the mysterious wasting-away of the the world's bee populations "colony collapse disorder." Researchers have come up with numerous hypotheses to explain the collapse, including parasites, viruses, and — yes — insecticides, but quantifying the impact of these and other suspects has proven to be an onerous task.

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Two studies, which are published in this week's issue of Science, opted to take a closer look at a class of insecticides called neonicotinoids. Neonicotinoids have been linked to colony collapse in the past; the incredibly effective pesticide works by spreading throughout the seeds and stems of treated plants; making its way into nectar and pollen supplies, where it is picked up by pollinators like bees; and, finally, attacking the nervous system of the exposed insects.

In the first study, researchers exposed 75 bee colonies to a neonicotinoid called imidacloprid (a pesticide whose use is authorized for over 140 crops in 120 countries worldwide) over the course of 14 days. Bees were exposed to either a low dose or a high dose, but both doses were considered to be "field-realistic," meaning they were administered at levels that you would expect to find in a field of treated crops. The colonies were then placed in a field and allowed to forage freely for a total of six weeks. When the six weeks were up, the researchers found that colonies exposed to low and high doses of imidacloprid were 8% and 12% smaller, respectively, than colonies that hadn't been treated with pesticide whatsoever. But the real shock came when the researchers realized that the treated colonies had produced 85% fewer queen bees than the untreated groups. In other words, for every ten queen bees an untreated colony produced, an imidacloprid-treated colony would produce just one or two.

In the second study, scientists tracked the comings and goings of individual bees to and from their hives by tagging them with tiny radio-frequency identification (RFID) chips (pictured here is an RFID-labeled bee and an RFID-reader-equipped hive). They researchers discovered bees that had been dosed with thiamethoxam (another popular neonicotinoid) were more than twice as likely as untreated bees to die while away from the hive, due to what they describe as "homing failure." In other words, the thiamethoxam seems to impede the bees' navigational abilities. Computer simulations later suggested that these wayward bees were getting lost often enough to cause hive populations to crash.

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The two studies complement each other well. The wayward tendencies of the bees in the second study, for example, could help explain the decrease in productivity observed in the first. The question now, however, is how pesticide companies will respond. On one hand, these two papers provide some of the most compelling evidence yet that neonicotinoids play a significant role in the death of bees around the world; and yet, most researchers agree that pesticides alone cannot account for the declines we're seeing.

"There are a whole lot of things that stress the honeybees," said Eric Mussen, a honeybee specialist at UC Davis, in an interview with the LA Times. "You can't point your finger at one thing and say, 'That is the problem.'"

Other factors could include climate change and disease, but given that bees pollinate an estimated 33% of America's agricultural output alone, is that any excuse to postpone the regulation — or at least thorough re-evaluation — of neonicotinoids and other pesticides?

Both papers can be read in this week's issue of Science (here and here).Top image via; RFID-bees via Science/AAAS